Variable optical attenuators are used to attenuate light beams in optical systems such as fiber optic communication systems. Controllably attenuating a light beam can be achieved by various methods, such as by passing the beam through a variable-attenuation (e.g. wedge-shaped) filter, radially bending a fiber loop to vary the optical loss within the loop, partially blocking the beam by inserting a beam block into the light beam path, and moving a mirror relative to one or more fibers to alter the efficiency of light coupling to/from the fibers.
In U.S. Pat. No. 6,304,709, Fujita describes a variable optical attenuator comprising two parallel fibers with their end faces positioned facing a planar mirror. A convergent lens is placed between the fiber end faces and the mirror. A light beam emitted by the first fiber is reflected by the planar mirror back into the second fiber. To vary the optical attenuation imparted by the attenuator, the mirror is moved linearly so as to vary the distance between the mirror and the two fiber end faces. Other variable optical attenuators employing parallel fibers facing a reflective surface are described for example by Robinson in U.S. Pat. No. 6,137,941, and Takahashi in U.S. Pat. No. 6,483,982.